Nitriding method

ABSTRACT

METALIC WORK PIECES, PARTICULARLY ALLOWED STEEL WORK PIECES ARE NITRIDED AT TEMPERATURES ABOVD 500*C AND ARE THEN CONVEYED INTO A PRESSURE VESSEL AT SUCH SPEED THAT DURING CONVEYING OF THE WORK PIECE THE TEMPERATURE THEREOF WILL DROP RELATIVELY LITTLE, NOT MORE THAN 100*C AND PREFERABLY SO THAT THE WORKPIECE WHEN INTRODUCED INTO THE PRESSURE VESSEL STILL HAS A TEMPERATURE OF A LEAST ABOUT 500*C. PROMPTLY AFTER INTRODUCTION OF THE HOT WORK PIECE. THE PRESSURE VESSEL IS EVACUATED AND THE HOT WORK PIECE IS ALLOWED TO COOL BY HEAT RADIATION WHILE BEING LOCATED IN THE EVACUATED PRESSURE VESSEL. COOLING IN THE ABOVE DESCRIBED MANNER WILL IMPROVE THE QUALITY OF THE NITRIDED WORK PIECE.

Feb. 2, 1971 H, H i N 3,560,271

NITRIDING METHOD Filed May 16, 1968 United States Patent F Int. Cl. C23c9/14; C21d 1/74 US. Cl. 148-155 7 Claims ABSTRACT OF THE DISCLOSUREMetallic work pieces, particularly alloyed steel work pieces arenitrided at temperatures above 500 C. and are then conveyed into apressure vessel at such speed that during conveying of the work piecethe temperature thereof will drop relatively little, not more than 100C. and preferably so that the workpiece when introduced into thepressure vessel still has a temperature of at least about 500 C.Promptly after introduction of the hot work piece, the pressure vesselis evacuated and the hot work piece is allowed to cool by heat radiationwhile being located in the evacuated pressure vessel. Cooling in theabove described manner will improve the quality of the nitrided workpiece.

BACKGROUND OF THE INVENTION The present invention is concerned with thenitriding of metallic work pieces, particularly work pieces of highlyalloyed metals and primarily work piecesof alloyed steel, whereby thesteel may be alloyed, for instance, with one or more of the metalsnickel, manganese, chromium, molybdenum, vanadium and tungsten.

Such alloyed steels are used for tools and also for machine parts whichmust withstand sliding contact, such as crankshafts, pistons andcylinders in which pistons are to move. Such products are progressivelymore sensitive with respect to crack formation the higher the proportionof alloy constituents therein. For this reason, as will be discussedfurther below, the advantages of the present invention are particularlysignificant with respect to tools and work pieces of highly alloyedmetals. However, the invention is not necessary limited to the treatmentof highly alloyed steel. For instance, the present invention is alsohighly advantageous in connection with the nitriding of stellite, i.e.an alloy of predominantly chromium and cobalt.

Nitriding may be carried out in various manners known per se in the artand the present invention is particularly concerned with modificationsof the nitriding, particularly the subsequent cooling of the nitridedwork piece, in such cases in which the nitriding is carried out inmolten nitriding salt mixtures, for instance by the method known as theTenifer method of the firm Degussa, Germany. This method comprisesmelting a mixture of nitrogen-containing salts and immersing the workpieces which are to be nitrided into the molten salt bath.

This and other conventional nitriding methods permit excellent hardeningof alloyed metals, particularly highly alloyed steel, whereby the workpieces also may be formed of sintered metals. Contrary to theconventional cementing methods, such nitriding gives particularly highhardness values; however, the depth or penetration of the thus-formednitride layer is rather limited, especially in the case of highlyalloyed metals even if the nitriding is carried out for prolongedperiods of time. The depth of nitriding may reach values of barelyhundred microns; however, this depth referes to the depth of diffusionof nitrogen and not to the depth to which actually a nitrogen-containingmetal compound such as Fe N is formed.

'ice

This compound zone generally has a thickness of only about l020 microns.The exact composition of the compound zone is of no concern here, but itis important to note that there is formed by nitriding a relatively thincompound zone, much thinner than the zone of diffusion, which compoundzone is mainly responsible for the hardness of the treated work pieceand may consist for instance of a complex compound of cementite andnitride such as Fe C-Fe N. At a greater depth, i.e. within the remainingdiffusion zone, nitrogen atoms will be found but not the alloy orcompound Fe N.

It is an important feature of nitriding that the abovedescribed compoundzone is so very thin, i.e. generally has a thickness of only to up toabout 20 microns.

Due to the small cross-sectional dimensions of the hard, outer compoundlayer or zone formed during the nitriding in molten nitriding salt, thethin hard layer is very critical and cannot be recreated by welding uponadditional material the compound zone being a complex combination withnitrogen. Moreover during cooling of the hot nitrided work pieces aconsiderable risk exists that for the reasons discussed above tensioncracks will be formed. Such tension cracks may occur during cooling andalso immediately after completion of the hot nitriding, depending on theocnditions under which the cooling is carried out, for instance in awater or oil bath.

More moderate cooling by exposure to air cannot be carried out since, asis well known, upon cooling in contact with air damaging surfacecorrosion phenomena occur.

However, frequently, such cracks are formed only after putting thenitrided work piece, for instance a tool, into operation. For instance,in dies for extrusion presses used for producing profiled light metalbuilding elements which have relatively complicated cross-sectionalconfigurations, considerable stresses will be found, particularly incorner portions of the die profile which even under a low load or aftera short period of operation will cause crack formation and therebyrender the die unusable.

It is thus most desirable to carry out the cooling of the hot nitridedwork pieces in a moderate and protective manner; however, up to now nopractical and fully satisfactory solution has been found for thisproblem. Particularly, it is not possible to reduce the undesirabletensions by carrying out the cooling of the freshly nitrided hot workpiece by blowing air or hot air against the Work piece. This will retardthe speed of cooling; however, the salt-holding work piece surfaces tendto react with the oxygen of the air with formation of firmly adheringsurface layers consisting essentially of the thus-formed reactionproducts. Subsequent surface treatment of the cooled work piece is notpossible since it would reduce the crosssectional dimension of the hardcompound zone which, as pointed out above, is very small to begin with,and thereby the useful life span of the thus-nitrided and cooled workpiece would be at least considerably reduced.

Thus, the problem of cooling the freshly nitrided work pieces which inliquid nitriding salt solutions generally were heated to a temperatureof somewhat below 600 C., such as 570 C., represents a very seriousproblem. The nitrided work pieces are highly sensitive against surfacecorrosion by reaction with oxygen and against the formation of tensioncracks. Particularly if the hard nitrided compound zone at the surfaceof the work piece is combined with complicated configurations, heattensions occur as soon as the work piece is taken from the molten saltbath and subjected to cooling. If cooling is carried out suflicientlyslowly so as to avoid such heat-caused tensions, then a very harmfulcorrosion layer will be formed on the surface of the hard compound zoneof nitrided metal which corrosion layer can only be removed with verygreat difficulties, if at all, for instance if the work piece is ofcomplicated shape as for instance indicated by reference numeral 21 inFIG. 2 of the drawing.

On the other hand, if cooling is carried out at a faster rate so thatthe corrosion risk is reduced, then the heat tensions will not beequalized or balanced and crack formation or even breaking of the workpiece may occur.

Thus, up till now no practically successful method did exist fornitride-hardening of work pieces of certain complicated surfaceconfigurations since nearly always cracking of the metal occurred whenit was attempted to avoid excessive corrosion of the surface.

It is an object of the present invention to provide a method and deviceor arrangement which will overcome these difficulties and will result inobtaining cooled nitrided work pieces which are not excessively corrodedand in which the interior heat tensions are nearly completely equalized.

Furthermore, even if in conventional manner the hot nitrided work pieceswere cooled by immersion in a suitable cooling liquid and thus excessivecorrosion of the surface by contact with the oxygen of the air has beenavoided, nevertheless, lapping or wet-honing is necessary and thiscleaning of the surface generally has to be carried out in a series ofsteps and will not be effective with respect to surface portions whichare inaccessible or only diflicultly accessible.

However, the removal of corrosion layers which are formed even uponcooling in liquid media (thereby risking crack formation) is practicallyimpossible with respect to, for instance, the interior surface portionsof an extrusion die. The removal of such corrosion layers, however, isnecessary and important if upon operation of the die in an extrusiondevice, extruded bodies of the desired smooth surface configuration areto be obtained.

All these measures do not only requre substantial labor but are alsoconnected with the risk that the hard nitrided surface layer of therespective work piece will be damaged thereby and thus would have to befurther worked. Frequently, hardened work pieces with damaged surfacelayers which can no longer be reconditioned represent a valueless orpractically valueless waste material.

It would be possible to consider prevention of formation of corrosionlayers by maintaining the work pieces after hot nitriding and until theyare cooled to, for instance, room temperature, in an inert atmospheresuch as a nitrogen atmosphere. However, the costs and difiiculties incontrolling the individual steps of the process if the same has to becarried out in a nitrogen or the like atmosphere have been found to bepractically insurmountable so that the cooling in a nitrogen atmospherein which the hot work piece would have to be maintained upon re- *movalfrom the nitriding bath until it is sufiiciently cooled, did not meetwith practical success.

The same holds true for operating in a vacuum which possibly could bedone under laboratory conditions, but would not lend itself toindustrial operations, considering the weight of the work pieces, thenecessity of maintaining the molten nitriding salt bath and theconveying devices for removing the work piece therefrom as well as theresting place for the work piece during cooling thereof, under suchreduced pressure and thereby in a substantially oxygen-free atmosphere.

It is therefore an object of the present invention to overcome theabove-disscussed difficulties and disadvantages, to simplify the coolingof hot nitrided work pieces, particularly work pieces which werenitrided by immersion in a molten bath of nitriding salts, so that evenin the case of work pieces of complicated surface configurationtension-crack formation is substantially avoided and at the same timethe formation of harmful surface layers such as the above-describedcorrosion layers or the like will be substantially or fully eliminatedso that subsequent working of the surface of the cooled nitrided workpiece by lapping or other cleaning methods can be reduc d to a minimum.

4 SUMMARY OF THE INVENTION According to the present invention, metallicwork pieces are nitrided by subjecting the work piece to nitriding at anitriding temperature which is significantly above about 500 C.,generally somewhat below 600 C. and frequency about 570 C. Thethus-nitrided hot work piece is then conveyed through the ambientatmosphere into an evacuatable zone at such speed that during passagethrough the ambient atmosphere the temperature of the hot nitrided workpiece will drop only slightly, generally less than about C. andpreferably not below about 500 C. Immediately after the work piece hasbeen placed into the evacuatable zone, the latter is evacuated to aresidual pressure of up to about 20 mm. of mercury, preferably tobetween 2 and 20 mm. mercury, or up to 2 mm. mercury, and the still hotWork piece, which has been introduced into the evacuatable zone at atemperature preferably above or not lower than about 500 C., will thenbe allowed to cool by heat radiation while located in the thus-evacuatedzone.

The metallic work piece may be and frequently is formed of higfhlyalloyed steel and the cooling in the evacuated zone may be continueduntil the work piece has reached room temperature, but generally it willsufiice to continue cooling by heat radiation of the work piece locatedin the evacuated zone until the temperature of the work piece has beenlowered to about 0, since at a temperature of about 150 C. practicallyno reaction between the surface layer of the work piece and the oxygenof ambient air would occur.

The evacuated zone preferably is represented by the interior of apressure vessel into which the work piece is introduced, whereupon thepressure vessel is closed and evacuated.

Since the heat radiation emanating from the work piece would eventuallylead to heating of the wall of the pressure vessel and thus to causing acounter-current heat radiation from the walls of the pressure vesseltowards the work piece, thereby possibly unduly retarding the coolingprocess, it is frequently preferred to cool the walls of the pressurevessel, or portions of the walls thereof in conventional manner, forinstance by spraying the outside of the pressure vessel with water orarranging cooling coils thereon of any other of the many methods knownfor this purpose to those skilled in the art.

In any event, the heat withdrawal from the hot work piece takes place byheat radiation therefrom while the work piece is located in theevacuated interior of the pressure vessel.

The present invention is also concerned with an arrangement for carryingout the above described method which arrangement comprises a nitridingfurnace or vessel in which a molten bath of nitriding salts ismaintained, the above-described pressure vessel, suitable conduits and avacuum pump or the like for evacuating the pressure vessel, conveyingapparatus for lifting the nitrided hot work piece from the molten saltbath and introducing it into the pressure vessel, and electronic controlarrangements for actuating the conveying arrangement, the vacuum pump orthe like, and possibly also for opening and closing the lid of thepressure vessel in accordance with a preset schedule which may be madedependent on readings of time and temperature.

Since in many cases the length of the cooling period may exceed the timerequired for nitriding the work piece, it is also within the scope ofthe present invention to provide an arrangement in which a plurality ofpressure vessels is combined with a lesser number, or one, nitridingfurnace or molten bath of nitriding salts so that the nitriding bath orfurnace may be used to its full capacity by successively feeding the hotnitrided work pieces into different ones of the series of pressurevessels arranged in the vicinity of the nitriding furnace. The conveyingof the hot nitrided work pieces to specific ones of the series ofpressure vessels also may be electronically controlled in a mannerwhich, per se, will be apparent to those skilled in the art.

The novel features which are considered as characteristic for theinvention are set forth in particular in the appended claims. Theinvention itself, however, both as to its construction and its method ofoperation, together with additional objects and advantages thereof, willbe best understood from the following description of specificembodiments when read in connection with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic, elevational,perspective view of a complete nitriding arrangement in accordance withthe present invention; and

FIG. 24 are illustrations, on a scale larger than that of FIG. 1, ofspecific work pieces which may be advantageously nitrided in accordancewith the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In connection with the solutionof the above-discussed problems in accordance with the presentinvention, two basic concepts should be considered.

With respect to the danger of formation of tension cracks, it is quiteclear that this danger depends on the type and manner of the coolingprocess to which the hot, freshly nitrided work pieces are exposed,whereby generally heat is given up by the work pieces by heat conductionand convection. Heat loss by radiation may be disregarded inasmuch as,in accordance with Bolzmanns Law, the radiation of heat is proportionalto the fourth power of the respective temperature differential and thuswill be important only during the initial cooling of the still ratherhot work piece, and it is well known that in this temperature range,i.e. while the work piece is still at a temperature relatively close tothe nitriding temperature, heat cracks generally do not occur.

Consequently, the withdrawal of heat and thus the cooling of the workpiece by radiation would be particularly desirable and even more soduring progressive cooling since heat radiation emanates essentially inradial direction, i.e. excentrically relative to the work piece and,consequently, the interior configration, particularly interior surfacesof the work piece play only a relatively unimportant role.

Work pieces with complicated inner surfaces, such as extrusion dieshaving inner surfaces which in part are outwardly flaring, would beseverely endangered by cooling in a medium such as gas or liquid whichwould directly contact such inner surface portions. On the other hand,the zone of these inner surface portions would participate only veryslightly or practically not at all in the initial heat withdrawal byradiation since in this case, between closely adjacent inner surfaceportions, even desirable heat or temperature equalizations would occurand consequently absorption or reflection of heat rays would to aconsiderably extent compensate each other. Thus, a work piece with suchcomplicated interior surface configurations would radiate approximatelythe same amount of heat into the surrounding space as a simple solidmetal body of similar outer shape and surface. The speed of cooling orthe period of time required for cooling of work pieces of equal mass,which by itself is not of particular importance, will be nearly the sameirrespective of differences in the surface configuration, particularlythe configuration of inner surfaces, of the work pieces.

The second concept which is to be considered in connection with thepresent invention, is concerned with the formation of undesirablesurface or corrosion layers. The formation of surface layers takes placenot so much at the point or moment of withdrawal of the work piece fromthe molten salt bath and its exposure to the ambient atmosphere but onlysomewhat later, namely after the oxygen of the air had time to diffusethrough the salt layer adhering to the surface of the work piece andthus to come into direct contact with the metal surface of the workpiece. It has been found that harmful surface layers are formed by theeffect of contact with the ambient atmosphere only after the work piecehad been removed from the molten salt bath for a certain length of time.

It thus has been surprisingly found in accordance with the presentinvention that it is possible to expose the work piece immediately afterremoval of the same from the molten salt bath, for a limited period oftime, provided that the contact between the work piece (with a saltlayer adhering thereto) and the ambient atmosphere is effectivelyinterrupted after a relatively short period of time.

It would seem that these two concepts cannot be coordinated since thedesired retardation of cooling for the purpose of avoiding tension-crackformation necessarily requires maintenance of relatively hightemperatures and thus greater reactivity for prolonged periods of time,particularly since the use of inert gases as cooling fluid does notrepresent an economically feasible solution of this problem.

However, in order to approach the theoretically desired or idealconditions and to obtain the technical advantages of cooling of the workpiece by heat radiation at reduced surrounding pressure as well as toavoid chemical surface reactions by substantially exclusion of oxygenand moisture in a relatively simple and economical manner, it isproposed according to the present invention to convey the work pieces,as they are withdrawn from the nitriding molten salt bath, without anycontrol of temperature or chemical reactions, but without substantialdelay, through the ambient atmosphere into a closable pressure vessel,and thereafter to evacuate the pressure vessel to a residual pressure ofat most about 20 and preferably about 2 mm. mercury, whereupon coolingof the work piece will take place by heat radiation while the work pieceis located in the evacuated vessel.

Surprisingly, it has been found by proceeding as outlined above, thatthe cooling by convection which takes place during the relatively shortperiod of time during which the work piece is transported in contactwith the ambient air from the nitriding vessel to the pressure vesselwill not increase the risk of the formation of tension cracks ascompared with laboratory experiments in Which the work piece wasmaintained in a partial vacuum from the moment of leaving the nitridingbath until sufliciently cooled.

By proceeding in this manner discards were no longer obtained, i.e. workpieces which were unusable due to formation of tension cracks, and thusit became possible to nitride work pieces of very complicated surfaceconfigurations, particularly inner surface portions, in a much morereliable and effective manner than was believed possible up to now.

According to the present invention, the work pieces have to be conveyedfrom the hot salt bath into the pressure vessel. It is desired to carryout this conveying as quickly as possible. However, no harm is done ifduring such conveying the work piece is cooled by up to about C. At thebeginning of the conveying, the surface of the work piece is stillcovered by salt and, consequently, there is little risk of corrosion.Furthermore, obviously, at the beginning of the cooling no interior heattensions will occur and this also is essential in accordance with thepresent invention.

Consequently, it is possible to expose the work piece for a limitedperiod of time during the conveying from the molten salt bath to thepressure vessel to the influence of the ambient atmosphere. The exactlength of time for which the work piece may be exposed to the ambientatmosphere prior to being subjected to reduced pressure in the pressurevessel, and the extent to which the work piece may be cooled during suchtransfer and prior to being exposed to the reduced pressure, will dependon 7 the size and mass of the work piece and to some extent on themanner in which the entire nitriding facilities are arranged. Smallerwork pieces may be more quickly conveyed into the pressure vessel and,nevertheless, the temperature of such smaller work pieces may drop byabout 100 C. prior to being exposed to reduced pressure.

On the other hand, large and heavy work pieces, for instancecrankshafts, cannot be conveyed with such speed and thus it may take upto about one minute or so from the withdrawing of the work piece fromthe molten salt bath until the work piece is subjected to the pressurereduction. However, due to the greater mass of such larger work piece,the cooling during the relatively prolonged period of time, namely aboutone minute, may be as little as 50 C.

The preferred temperature of molten salt nitriding baths is about 570 C.and it is desirable that the temperature of the work piece upon beingintroduced into the pressure vessel, i.e. immediately prior toevacuation of the pressure vessel, should still be somewhat above 500 C.To expose the work piece to the reduced pressure while the temperaturethereof is still above 500 C. is particularly important in the case ofcomplicated surface configurations.

On the other hand, a relatively simple work piece such as that shown inFIG. 2 and formed with a simple central bore instead of the ratherinvolved configuration of the aperture indicated by reference numeral21, possibly may be completely cooled while exposed to the ambientatmosphere. However, in such case the work piece would obtain a veryrough surface due to the above-discussed chemical reactions which wouldtake place upon such prolonged exposure during cooling to the influenceof the oxygen of the ambient atmosphere and it would be most expensiveand difiicult to subsequently convert such surface into the desiredsmooth surface.

Surprisingly, it has been found that by proceeding in accordance withthe present invention the surface of the cooled work piece remainssmooth and substantially free of corrosion or undesirable surfacelayers. It had been expected that due to the reduced pressure to whichthe work piece is exposed within a few minutes after being removed fromthe nitriding bath, the surface conditions of the work piece would beimproved.

However, it was still surprising to find that even after completion ofthe retarded cooling in the evacuated pres sure vessel, work pieces,which, for instance, had been retained in the evacuated pressure vesselfor about five hours, had a perfectly smooth surface which, afterrinsing off of residual salt, in many cases did not require any furtherworking. This seems to be proof of the contention that during theinitial short exposure of the hot work piece to the ambient atmospherethe then still present salt layer prevented any significant surfacecorrosion.

The method of the present invention has been successfully carried outwith respect to work pieces of greatly varying configurations and sizes.

Thus, the theoretically achievable advantages of the prohibitivelyexpensive complete vacuum treatment (i.e. vacuum treatment from themoment of withdrawal from the molten salt bath until cooling to asufficiently low temperature) may be achieved in an extremely simple andeconomical manner by proceeding in accordance with the present inventionwhich requires only the evacuation of a pressure vessel and not theevacuation of the entire working area, i.e. the entire path along whichthe work piece must move from withdrawal from the salt bath to beingplaced at rest in the location where cooling is completed.

It is a further advantage of the present invention that generally nofurther treatment of the work pieces is required, not only for removingcorrosion layers and the like, but also that high gloss polishing of thework piece surface can be carried out in a much more simple manner thanwas heretofore possible and it is noteworthy in this connection that thesurface of the cooled work piece will have the same dimensions as thefreshly nitrided work piece and the hard nitrided outer zone will not bedamaged since polishing, can be carried out substantally without theremoval of material.

Additionally, the method of the present invention lends itselfexcellently to substantial automation and electronic control whereasconventional methods of this type required substantial skilled labor forcontrolling the cooling process.

The work pieces will cool in the evacuated pressure vessel at a ratedepending on the mass of the respective work piece whereas the shape ofthe work piece will be of relatively insignificant importance withrespect to the cooling speed in accordance with the present invention.Thus, no special measures have to be taken in order to find out in anygiven case the most advantageous cooling rate which will avoid theformation of tension cracks.

Apart from substantially completely eliminating waste due to crackformation, and substantial reduction of corrosion, the labor costs ofthe process are substantially lower than that of conventional processessince not only the nitriding in the molten salt bath, but also thecooling of the hardened work piece can be easily and reliably controlledin an automatic manner.

Although completely different conditions prevail in the nitriding ofwork pieces of different alloy composition, different mass and differentshapes, no complications arise when proceeding in accordance with thepresent invention, and the work pieces may be nitrided for the desiredlength of time and then cooled, which up till now in the case of certainwork pieces of particularly complicated configurations could not besuccessfully accomplished due to corrosion layers and crack formation.Once the required time for nitriding has been determined and the timefor cooling by radiation which depends primarily on the mass of the workpiece, it is easily possible to automate the process or to subject it toelectronic controls. For instance, a switching drum or cam arrangementmay be used for controlling all required time intervals. Therequirements for skilled labor in carrying out the process of thepresent invention are thus greatly reduced.

The easy control and possible automation of the process of the presentinvention also facilitates the scheduling of the process since workintermissions do no longer pose a problem. For instance, nitriding inthe salt bath may be carried out up to the lunch break or up to the endof the working week and the cooling of the nitrided work pieces in theevacuated pressure vessels will then proceed during the break, week-endor the like, without requiring supervision.

While the present invention has been primarily described in connectionwith salt bath nitriding, it may also be used for the cooling of workpieces which were subjected to nitriding in a hot nitrogen atmosphere,whereby it will not be necessary to subject the nitrogen atmosphere tocooling.

In either case, the apparatus requirements are very limited,particularly since an oil pump for evacuating the pressure vessel may beelectronically controlled so as to stop operation when a desired minimumpressure of, for instance, 10 mm. mercury has been reached and,furthermore, one oil pump may serve for the evacuation of a plurality ofpressure vessels.

By utilizing packings of synthetic material which per se are known tothose skilled in the art for achieving an hermetic seal between the lidand the body of the pressure vessel, it is possible to maintain theinitially obtained degree of vacuum for prolonged periods of time whichsuffice for the cooling of the hardened work piece, particularly sincetemperatures as high as about C. are harmless and thus, the work piecemay be exposed to ambient temperature when it has been cooled to about150 C.

Referring now to the drawing and particularly to FIG. 1, it will be seenthat the arrangement according to the present invention comprises amolten salt bath furnace 1 of conventional construction with therequired auxiliary devices, a conveying device 2 and a pressure vessel 3which is connected with an electrically operated vacuum pump 4.

Salt bath furnace 1 contains the molten salt mixture of alkali metalcyanate and alkali metal cyanide as required for carrying out nitridingin accordance with the Tenifer method.

The conveying device 2 includes a rail on which conveyor elements 5 movein a manner, i.e. at given time intervals, which may be electronicallycontrolled. At the desired moment, work pieces 6 which had been immersedin furnace 1 are lifted with the help of servo motors by reducing thelengths of ropes 7, conveyed to pressure vessel 3 and introduced intothe opened pressure vessel by lengthening rope 7.

In the interior of pressure vessel 3, supporting elements (not shown)are provided on which the work pieces 6 will come to rest so thatthereby the work pieces will be disconnected from ropes 7. Afterintroduction of the work pieces, lid 10 which is supported by shaft 9,the latter being capable of rotating about its own axis, will be placedin position above opening 11 of pressure vessel 3, and lowered so as toclose the pressure vessel. Thereafter, pump 4 is actuated and theinterior of pressure vessel 3 is quickly evacuated.

Between the rim of opening 11 of the pressure vessel 3 and lid 10, apacking 12 is arranged, preferably formed of synthetic materialcommercially available under the name Teflon. In order to avoidexcessive heating of the packing, preferably and in conventional mannerwater cooling is installed in the side wall and/ or top of pressurevessel 3.

Furthermore, a temperature-sensing instrument 13, such as a thermometer,extends inwardly through the wall of pressure vessel 3 for the purposeof indicating the heat radiation therein which, in combination with thevalue for the residual pressure in the pressure vessel, will give avalue which can be converted in a manner known per se to indicate thetemperature of the work piece at any given time. The weight of lid 10and the pressure differential pressing the lid against the packing willserve for hermetic sealing and quick evacuation of the pressure vessel.

Conduit 14 connecting pump 4 with pressure vessel 3 may also communicatein per se known and not illus trated manner with several additionalpressure vessels 3, whereby each of the pressure vessels will requireonly about two minutes for being evacuated down to a residual pressureof about 15 mm. mercury.

After such evacuation, the further cooling of the work pieces in therespective pressure vessels does not require any supervision andgenerally will require several hours, depending on the shape andprimarily the mass of the respective work piece.

Preferably, as a further safeguard, there is also provided a valve 14which may be actuated in conventional magneto-electrical manner.

The specific electronic control devices utilized in connection withoperating the arrangement of FIG. 1 are per se conventional and notillustrated. They operate inexpensively and reliably.

The pressure vessel 3 as illustrated has a capacity of 280 liters andpump 4 a capacity of 60 cubic meters per hour. When it is desired toevacuate the pressure vessel further, down to a residual pressure ofabout 2 mm. mercury, it is still possible with one pump 4 to operate sixpressure vessels 3 without experiencing any difficulties.

It is also possible to arrange thermometer 13 so that the same will bein direct contact with the work pieces within the pressure vessel. Thismay be accomplished by resiliently mounting thermometer 13 and willoperate 10 satisfactorily since no quick movements of the work pieceswill take place.

The periods of time during which work pieces should be allowed to coolby heat radiation while located in the evacuated pressure vessel willdepend primarily on the weight or mass of the work piece and differ onlyvery little with respect to the specific composition or shape thereof.

Generally, it has been found that the desired results, i.e. a cooling tobelow 200 and preferably to about C., will be achieved with work piecesof 5 kilograms in about 2 /2 hours, weighing 8 kilograms in about 3 /2hours, weighing 18 kilograms in about 6 /2 hours and weighing 25kilograms in about 9 hours.

After such cooling times, the work piece surface was found to havecooled down by heat radiation to about 150 C. and removal of the workpiece from the pressure vessel and exposure of the work piece to theambient atmosphere was then possible without any risk of damage to thesurface of the work piece or of the formation of tension cracks.

In theory, the interior dimensions of the pressure vessel are of noconsequence since heat radiation emanates radially from the work piecein all directions. True radiation will occur as long as the surroundingsare colder than the work piece. For this reason, it is desirable to usepressure vessels which are provided, in their walls, with conventionalwater cooling. In this manner, the cooling period may be reduced toabout one-fifth of that achieved without water cooling, particularly incases in which the volume of the work piece or work pieces issubstantial relative to the inner volume of the pressure vessel 3. Thetemperature of the pressure vessel walls may be maintained by watercooling at about 20 C. so that initially a temperature differential ofnearly 500 C. will exist between the temperature of the work piece andthe temperature of the walls of the pressure vessel. Without such watercooling, the walls of the pressure vessels would warm up and wouldproduce heat radiation in the direction towards the work piece so thatthe temperature differential would be less than 50% of that achievedwith water cooling. Quite obviously, the temperatures of the work piece,as well as of the vessel walls, change during the cooling and theforegoing figures are given only by way of a rather extreme example. Forthe cooling of very expensive work pieces, or work pieces which areparticularly sensitive to cracking, water cooling is reduced oreliminated and thereby the cooling period is prolonged, generally up toabout 10 hours. In this case, only time but no additional work or energyis expended.

The work pieces 6 may consist, for instance, of sintered bodies ofchromium-nickel steel or of highly alloyed tools of chromium steel orSS-steel or may be produced of other alloys in a great variety of sizesand shapes, and in all these cases, even after a useful life span of thenitrided work pieces of several months, no tension cracks were observed.Particularly tools for warm working such as pressure casting molds forcasting light or heavy metals can be treated successfully in accordancewith the present invention, whereby due to the fault-free surfacesadherence between tool and mold will not take place. It was interestingto note thereby that the corrosion layer formed on the surface of thethus-treated tools could be eliminated in an even more simple mannerthan corrosion layers formed on work pieces which were cooled inconventional manner by quenching in oil. Since water quenched tools havean even greater surface roughness, it was found that, notwithstandingthe prolonged cooling periods, the work pieces which were cooled inaccordance with the present invention had optimum surfacecharacteristics.

It is well known that the surfaces of nitrided work pieces may beimproved, i.e. may be smoothed by being sub jected for a short period oftime to dull lapping with silicon carbide having a particle size passingthrough 800 mesh. In this manner, the residual roughness can be reducedto between 1 and 2 microns. If in addition a polishing lapping withglass pearls is carried out, then the thus-treated surface obtains asilken smooth appearance, whereby only fractions of 1 micron of thesurface layer are removed.

It 'was found that by proceeding in accordance with the presentinvention, the work pieces 6 which are removed from pressure vessel 3had in all cases a thin grey corrosion layer of surprisingly lowstability. It was found that the dull lapping with hard silicon carbideswas not necessary and that the polishing lapping with a dispersion ofglass pearls in water or the like fully suffices. Thereby, thehardness-providing compound surface zone remains completely undisturbed.Practical experience has shown that drawing tools which are exposed to agreat degree of stress when hardened in accordance with the presentinvention, possessed a substantially prolonged useful life span. Toolshardened in accordance with the present invention and then used for theheat deformation of light metals could be used without any subsequentsurface treatment, after washing off of the salt layer and removingresidual impurities with a soft brass brush.

It is a further advantage of the present method as compared withquenching in oil that the washing of the nitrided work pieces which weretreated in accordance with the present invention can be carried outsimply with cold water.

It is particularly important and significant that by proceeding inaccordance with the present invention only a very thin corrosion layeris obtained which layer, as has been described, can be easily removedwithout damaging the important thin, hard compound layer of, forinstance, iron carbides and iron nitrides.

In the case of work pieces which were subjected to conventionalnitriding and cooling, very uneven corrosion layers are formed, thestructure of which depends to a considerable extent on the structure andcomposition of the alloy and which corrosion layers themselves aresubject to heat stresses. For this reason, such corrosion layers formedin conventional processes may partially break off during the cooling ofthe work piece and thereby an extremely rough and difiicultly workablesurface is formed. These disadvantages are not experienced whenproceeding in accordance with the present invention.

Referring now to FIG. 2, the same shows by way of example an extrusiondie 20 which may be treated in accordance with the present invention.The extrusion opening 21 which is cut into die 20 serves for theextruding of aluminum rods which are used as building elements forwindows, doors and the like. The complicated crosssectionalconfiguration of the extrusion opening frequently led to tension cracksand the surface of extruded aluminum alloy rods which were formed byextrusion through hardened dies of the type of die 20 frequently showfine longitudinal grooves, due to the fact that it is not possible toeffectively work the hardened surfaces defining the extrusion orifice21. However, by hardening such dies in accordance with the presentinvention, no tension cracks occurred, the hardly accessible innersurface areas could be easily cleaned and were found then to bepractically as smooth as prior to the hardening treatment.

FIG. 3 illustrates as a further example of a work piece a forging diewhich combined with a similar die serves for producing valve bodies andwhich also has been hardened in accordance with the present invention,in an arrangement as illustrated in FIG. 1.

The surfaces of the hardened work pieces were of the desired smoothnessand particularly the critical areas 31 as well as areas 32 remained ofaccurate size even upon prolonged use under high stress no tensioncracks occurred.

Similar results were obtained, for instance, by nitriding the crankshaftillustrated in FIG. 4 wherein particularly hardening of bearings 41, 41,41" and of crankshaft 12 bearings 43, 43 and 44 and 44 are ofimportance, although simultaneously also main shaft 42 and pinion 45were subjected to nitriding and hardening.

As pointed out further above, the cooling method of the presentinvention may also be used for the cooling of work pieces, especiallywork pieces formed of alloyed steel, which were subjected to nitridingin a hot nitrogen atmosphere and which thereafter may be introducedwithout substantial delay but in contact with the ambient atmosphere,into pressure vessel 3 and therein cooled, as described above, by heatradiation while maintained at a residual pressure of for instance 5 mm.mercury.

The following examples are given as illustrative only without, however,limiting the invention to the specific details of the examples.

Example 1 An extrusion die as illustrated in FIG. 2 is to be nitrided.

The die consists of alloyed steel containing 0.4% carbon, 5% chromium,1.5% molybdenum and 1% vanadium, and has a weight of 2.85 kg.

The diameter of the die is 13.8 cm. and the extrusion opening 21 isformed with an accuracy of :0115 mm. The surface roughness of the innerfaces of the extrusion opening does not exceed 2 microns. The radii inthe corners or edges of the extrusion opening 21 are smaller than 0.5mm.

This blank has been warm hardened. Its dimensions are sufficientlyaccurate and the surfaces defining the extr-usion opening 21 are highlysmooth. There are no cracks in the edge portions of the extrusionopening and it is therefore possible to use die 20 for extrudingtherethrough aluminum alloys into a shape corresponding to that ofextrusion opening 21.

However, the wear and tear on die 20 is very high, particularly ifrelatively hard aluminum-silicon-magnesium alloys are to be extruded.Even by taking greatest care with respect to extrusion temperature,extrusion speed and lubricating agents, the useful lifespan of die 20 israther limited so that the costs of providing the die will amount toabout 6% of the total cost of the aluminum bodies extruded therethrough,since aluminum alloy particles adhere on the die 20.

However, if such die is hardened in a nitriding bath the useful lifespanof the die can be increased four to sixfold without impairing of qualityof the extruded rods so that the die costs will then amount to not muchmore than 1% of the total costs of the extruded rods. However, thissaving is substantially lost because the costs of conventional nitridingof the die increase the costs thereof by 100%. Y

The high cost of nitriding are due to the following:

The nitriding bath consists of a molten salt mixture containing cyanidesand cyanates and having a temperature of 570 C. Such salt mixture iscommercially available under the trademark Tenifer. -Die 20 remains inthe molten salt bath for minutes and thereby, due to diffusion, an alloyor compound zone is formed having a thickness of about 20 microns andconsisting essentially of iron carbide and iron nitride.

This alloy zone is very hard but also very thin. The hardened die isremoved from the salt bath at a temperature of 570 C. It has to bequenched or at least quickly cooled since otherwise the surfaces wouldcorrode.

Cooling in an ambient atmosphere causes the formation of irregularcorrosion layers and a roughness of about 5 microns. Consequently, it isnecessary to work the thus-roughened surfaces. This is difficult andexpensive because the surfaces have now been nitrided. Furthermore, suchworking removes a substantial portion of the hard nitride layer. Inaddition, such cooling in ambient atmosphere is connected with thedanger of dimensional distortion due to interior heat tensions. Furtherworking by grinding is extremely difficult and causes removal of anotherportion of the hard layer.

Consequently, by proceeding in this manner, the desired dimensionalcorrectness of the die can no longer be maintained.

Cooling by quenching in 'water will give a better surface layer,however, the dimensional distortion will be excessively large.Furthermore, by quenching in water, more than 20% of the dies are ruinedby heat tension cracks. Such cracks cause immediately or after a veryshort period of time breaking of the die 20. Dies such as die 20 withcracks formed therein, which cracks primarily occur in the edges of theextrusion opening 21, are not usable. Such cracks cannot be subsequentlywelded since the material contains N. Obviously, it is not possible toextrude rods of the desired cross-sectional shape by utilizing a diewhich has a visible crack since the markings of the crack will be seenat the outer face of the extruded rod and furthermore since the Zone ofthe die in which the crack is located will quickly break so that theentire extrusion device will have to be stopped. All of thesedisadvantages result in considerable additional cost due to frequentinterruptions of the extrusion operation as well as due to loss of thelabor involved in the preparation of the unhardened die 20.

Consequently, the only practical method appears to be the quenching inan oil bath. However, it has been found that in the case of dies such asdie 20 of FIG. 2, a waste of between 11 and 12% will be caused due toheat cracks which sometimes are immediately apparent and sometimes arefound after very short use of the die. It is not possible to determinewith certainty by visual inspection whether or not the hardened die 20has fine cracks. within the surfaces defining the extrusion opening 21and even if no actual crack has been formed, it is possible that the diehas been dimensionally distorted by heat tensions so that furtherworking would be required.

For all these reasons, it was possible by quenching the nitrided die inan oil bath to prolong the useful lifespan of the die about six times,however, the production costs per useful hardened die were about 100%higher than for the unhardened die.

By proceeding in accordance with the present invention, it was possibleto avoid this increase in the costs of the die. It was found that bynitriding the die in a molten salt bath and then cooling as describedabove in accordance with the present invention, practically no wastematerial was formed and substantially 100% of the treated dies could beoperated for the above indicated prolonged period of time.

This was accomplished as follows:

The dies 20 were hardened as before for 80 minutes at 570 C. in anitriding salt bath and then removed therefrom. The thus-hardened dieswere simply transported through the ambient atmosphere into the pressurevessel 3 of FIG. 1 and hung into the same. The time required for theremoval of the hardened die from the salt bath to closing. lid ofpressure vessel 3 after insertion of the die into the pressure vesselamounted to about 30 seconds and the temperature drop during this periodequaled about 1.5 C. per second or a total of about 45 C.

Immediately after closing lid 10, vacuum pump 4 was actuated and therebythe drop in temperature by heat conduction and convection was quicklyeffectively reduced. After operating the pump for 2 minutes the residualpressure in the pressure vessel amounted to mm. mercury. During thisperiod of 2 minutes, the temperature loss at the surface of the hardenedwork piece was reduced from 1.5 C. to only about 0.035 C. per second sothat the average temperature loss during these two minutes amounted to023 C. per second.

Thus, upon establishing a residual pressure of 50 mm. mercury thetemperature of die was still higher than 480 C.

Further cooling took place nearly exclusively by heat radiation.

After 2 hours and 15 minutes, pressure vessel 3 was 14 opened and it wasfound that the die 20 located therein had a temperature of less than 200C. (estimated). The first measurement was made 12 seconds after openingof the lid 10 and at that time the surface temperature of the die,probably due to contact with the air entering the pressure vessel, wasless than C.

It is of course also possible to reduce ambient pressure by theintroduction of heated air, however, this has been found to beunnecessary for a variety of reasons.

First of all, obviously cracks were not formed during the initial dropin the temperature from about 570 C. to 480 C. and during this periodalso no dimensional distortion of any significance did occur.Furthermore, during these first 30 seconds plus maximum 2 minutes, i.e.until a residual pressure of 15 mm. mercury had been established, noappreciable corrosion took place at the still liquid salt-coveredsurface of the die, notwithstanding the contact with the surroundingambient atmosphere and the oxygen content of the latter.

Secondly, the critical temperature range of from 480 C. to below 200 C.was passed so slowly (within 2% hours) that obviously cracks could notbe formed by such slow cooling. Possibly, initial heat tensions wereequalized during this period and, most important, due to the practicallycomplete lack of an atmosphere, no further corrosion took place.

Finally, the somewhat quicker cooling from below 200 C. down towardsambient temperature which occurred upon removal of the die, wassufiiciently mild or slow so that no cracks could be formed and, due tothe relatively low initial temperature, i.e. below 200 C., no visibleenlargement of the corrosion layer took place. The existing corrosionlayer was of gray color, as usual, however, surprisingly, it was veryeasily removable. Very significant was the high degree of smoothness ofthe sur face which had a roughness of only between about 1 and 2microns. It was possible to use the thus-cooled die without any furthertreatment for the extrusion there through of profiled rods ofaluminum-silicon-magnesium alloys. It was found that only the first30-40 cm. of the extruded rod were of somewhat lesser smoothness, butafter extruding such relatively small length, the corrosion layer at thesurface of extrusion opening 21, i.e. the inner surface of die 20 wascompletely removed.

The result of proceeding in this manner was a cost reduction of morethan 50% as compared with previously produced hardened dies andfault-free production of high quality hardened dies. The total die costsamounted truded aluminum rods.

Example 2 The work piece illustrated in FIG. 3 of the drawing, i.e., aforging die, was subjected to the process of the present invention.

The weight of die 30 was 1.65 kg. and the die was formed of achromium-molybdenum steel of the type H13.

Hardening of the die was carried out as described in Example 1 byimmersing the die in the nitriding salt bath maintained at 570 C.

It was found that generally a cooling period of 2 hours will sufiice forwork pieces weighing less than 3 kg., unless so many individual workpieces are introduced into one pressure vessel 3 that due to the totalmass of cooling metal therein the cooling period was prolonged.

The pressure vessel 3 was provided with water cooling in its cylindricalwall. Tap water was passed in contact with the wall of the pressurevessel so that the outer temperature of the wall remained below ambienttemperature and the spent cooling water was heated to a ten-.- peratureof only between about 20 and 30 C.

While generally proceeding as described in Example 1, it was found as aparticular advantage in this case that particularly no dimensionaldistortion occurred during the cooling. Consequently, two exactlypre-fashioned drop forging dies 30 will have an excellent fit evenwithout working the surfaces of the dies after the hardening and coolingof the same. By using dies of the type illustrated in FIG. 3 forproducing valve bodies, coaxial products are obtained practically freeof fins so that only little further working of the pressed productproduced in such die will be required. The surface roughness of thecooled die was only between 1 and 2 microns and the thin gray corrosionlayer which was formed similar to the layer formed according to Example1, is in this case preferably removed by lapping, preferably wet lappingwhich may be carried out by spraying a dilute aqueous soap solutionagainst the surfaces 31. It can also be carried out by spraying adispersion of very small glass spheres in water. These various lappingmethods are well known to those skilled in the art.

Again it was noted that crack formation was completely absent, the hardnitrided layer was completely maintained since no material-removingfurther working was required. Consequently, the useful lifespan of thedrop forge dies was prolonged and thus also the number of valve bodiesthat could be produced with one pair of the dies.

Example 3 The crank shaft illustrated in FIG. 4 was subjected tonitriding and cooling in accordance with the present invention. Beforenitriding the forged shaft was warm hardened.

The crank shaft was produced by drop forging of the alloy identified as42CrMo4, and weighed 11.2 kg.

Hardening was carried out as described in Example 1 at a temperature of570 C. The residence time of the crank shaft in the salt bath was 90minutes and it was found that the thickness of the hard compound surfacezone was more than 20 microns.

Removal of the crank shaft from the salt bath and introduction into thepressure vessel 3 required 30 seconds and two minutes later a residualpressure of 15 mm. mercury was reached in pressure vessel 3.

Due to the greater size of crank shaft 40 as compared with the wonkpieces described in Examples 1 and 2, the extent of cooling duringtransfer of the work piece from the bath into the pressure vessel wassomewhat less and amounted only to 35 C. and to the point at which theresidual pressure of 15 mm. mercury was reached only to about 60 C. Theresidence time of the crank shaft in evacuated pressure vessel 3' was 4hours and upon subsequent removal the temperature of the crank shaft was160 C.

[No measureable dimensional distortion was found and therefore it wasalso not necessary to carry out any further Working. It was also notnecessary to subsequently grind any of the bearing faces. The corrosionlayer could be sufliciently removed by spraying with soap solution. Thesurface roughness was only 1 micron. The thuscooled crank shaft could beimmediately mounted without requiring any further treatment.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can by applying current knowledgereadily adapt it for various applications without omitting featuresthat, from. the standpoint of prior art, fairly constitute essentialcharacteristics of the generic or specific aspects of this inventionand, therefore, such adaptations should and are intended to becomprehended within the meaning and rang of equivalence of the followingclaims.

What is claimed as new and desired to be protected by Letters Patent isset forth in the appended claims:

1 1. A method of nitriding a ferrous workpiece, comprising the steps ofimmersing said workpiece in a molten nitriding salt bath at atemperature significantly above than about 100 C.; evacuating saidpressure vessel promptly after introduction of said workpiece into thesame to a residual pressure of up to about 20 mm. Hg; and allowing saidworkpiece to cool by heat radiation while located in the thus evacuatedzone and simultaneously cooling said pressure vessel to counteract theheating of the walls of the pressure vessel during cooling of theworkpiece.

2. A method as defined in claim 1, wherein said hot workpiece isconveyed from said nitriding into said evacuatable zone at such speedthat the temperature of said hot nitrided workpiece will drop duringsaid conveying by less than the difference between said nitridingtemperature and about 500 C. so that said 'workpiece will be introducedinto said pressure vessel at a temperature of at least about 500 C.

3. A method as defined in claim 1, wherein said metallic workpiececonsists essentially of steel.

4. A method as defined in a claim 1, wherein said pressure vessel isevacuated to a residual pressure of between about 2 and 20 mm. Hg.

5. A method as defined in claim 1, wherein said pressure vessel isevacuated to a residual pressure of up to about 2 mm. Hg.

6. A method as defined in claim 1, wherein said cooling of saidworkpiece in said pressure vessel by heat radiation is continued atleast until the temperature of said workpiece has been lowered to aboutC.

7. A method as defined in claim 1, wherein said nitriding temperature isabout 570 C.

References Cited UNITED STATES PATENTS 1,644,828 10/1927 Guibert 14816.71,953,647 4/1934 Darrah 148--15.5 1,961,520 6/1934 Malcolm 148,15.52,041,769 5/1936 Larkin 14815.5X 2,916,409 12/1959 Bucek 148--16.6X

CHARLES N. LOVELL, Primary Examiner

